As a technology that the inventors of the present invention have been examined, a semiconductor device using a connecting material will be described with reference to FIG. 1 and FIG. 2. FIG. 1 is a diagram showing a structure of a conventional semiconductor device. FIG. 2 is a diagram for explaining the flash caused by remelted solder.
As shown in FIG. 1, a semiconductor device 7 is manufactured by connecting a semiconductor element 1 onto a frame 2 by solder 3, wire-bonding an inner lead of a lead 5 and an electrode of the semiconductor element 1 by a wire 4, and then sealing these components by sealing resin 6 or inert gas.
The semiconductor device 7 is reflow soldered using Sn—Ag—Cu series medium-temperature lead-free solder to a printed circuit board. The melting point of the Sn—Ag—Cu series lead-free solder is as high as about 220° C., and it is supposed that a connecting portion is heated up to 260° C. at the reflow connection. Therefore, for the temperature hierarchy, high-lead solder having a melting point of 290° C. or higher is used in the die bonding of a semiconductor element in a semiconductor device. The high-lead solder contains lead of 85 wt. % or more as its constituent, and the environmental load of the high-lead solder is larger than that of Sn-Pb eutectic solder that has been prohibited by the RoHS directive that took effect in July, 2006. Therefore, the development of a substitute connecting material to replace the high-lead solder has been desired.
At present, the melting point of the Sn—Ag—Cu series solder that has been already developed is 260° C. or lower. Therefore, if the solder is used in the die bonding of a semiconductor element, it is melted in the secondary mounting (maximum temperature: 260° C.). In the case where a surrounding area of the connecting portion is molded with resin, when the inner solder is melted, due to the volume expansion in the melting, the so-called flash occurs in some cases as shown in FIG. 2, in which the solder 3 leaks from the interface between the sealing resin 6 and the frame 2. Alternatively, if not leaks, the action of leakage occurs, and as a result, a large void 8 is formed in the solder after its solidification, so that a defective product is produced. As candidates of the substitute material, Au series solders such as Au—Sn, Au—Si and Au—Ge, Zn and Zn—Al series solders, and Bi, Bi—Cu, and Bi—Ag solders have been reported, and further examination has been made all over the world.
However, the Au series solder contains Au of 80 wt. % or more as its constituent, and thus it lacks versatility in terms of cost. The Bi series solder has a heat conductivity of about 9 W/mK which is lower than that of the current high-lead solder, and it is supposed that the application thereof to a power semiconductor device, a power module and others requiring high heat dissipation characteristics is difficult. Further, although Zn and Zn—Al series solders have a high heat conductivity of about 100 W/mK, the solder wettability thereof is low (in particular, Zn—Al series solder) and the solder itself is hard, and there occurs a problem that a semiconductor element is frequently broken due to the thermal stress at the time of cooling after the connection.
In Japanese Patent Application Laid-Open Publication No. 2002-358539 (Patent Document 1) and Japanese Patent Application Laid-Open Publication No. 2004-358540 (Patent Document 2), by using an alloy consisting of Al of 1 to 7 wt. %, Mg of 0.5 to 6 wt. %, Ga of 0.1 to 20 wt. %, P of 0.001 to 0.5 wt. % and the balance of Zn, an alloy consisting of Ge of 2 to 9 wt. %, Al of 2 to 9 wt. %, P of 0.001 to 0.5 wt. % and the balance of Zn, or an alloy consisting of Ge of 2 to 9 wt. %, Al of 2 to 9 wt. %, Mg of 0.01 to 0.5 wt. %, P of 0.001 to 0.5 wt. % and the balance of Zn, the wettability of the Zn series solder alloy to Cu and Ni is improved and the melting point thereof is decreased. However, since Al and Mg are contained, an Al oxide film and an Mg oxide film are formed on a surface of the melting portion by the heating in the connection. Since these films decrease the wetness, there is the possibility that the sufficient wetness cannot be obtained unless the film is mechanically broken by scrubbing or the like. Further, since no improvement is achieved in the hardness of the solder, the improvement for the breakage of a semiconductor element due to the thermal stress at the time of cooling after the connection or in the temperature cycle cannot be expected.
In Japanese Patent Application Laid-Open Publication No. 2002-261104 (Patent Document 3), an In, Ag or Au layer is provided for an outermost surface of a Zn—Al series alloy, thereby suppressing the oxidation of the surface of the Zn—Al series alloy and improving the wettability. However, since processes such as plating and evaporation onto the Zn—Al surface are indispensable for providing the In, Ag or Au layer, the processes for manufacturing the material are increased. Similarly, though the hardness can be reduced by adding In, the effect enough to prevent the breakage of the semiconductor element due to the thermal stress at the time of cooling after the connection cannot be expected.